Modifying data in an encoded data signal has become a vital function in studio editing environments. A possible solution has been proposed in the international patent application WO 99/51033 (PHF98546). This patent application describes a method and its corresponding device for modifying data in an encoded data signal. This method allows an additional data signal insertion, e.g. a logo inserting, into an MPEG-2 bitstream thanks to bit rate transcoding. Logo insertion comes as an extension of the bit rate transcoder. The corresponding diagram, depicted in FIG. 1, comprises a transcoding module 101 and a logo addition branch 102. The general outline of the transcoding module 101, well known to a person skilled in the art, comprises:                a residue decoding branch 118 for receiving the input signal 125 and providing a decoded data signal Error_I(n). This branch comprises in series a variable length decoding 107, an inverse quantization 108 followed by an inverse discrete cosine transform 109.        a re-encoding/decoding branch 120 for providing the output signal 126 and its decoded version respectively. The re-encoding part, for providing said output signal, comprises in series a discrete cosine transform 110, a quantization 111, a variable length coding 112 followed by a buffer 113, and regulation means 114 ensuring a constant picture quality of the output signal 126, and a first subtracter 122 generating a coding error. The decoding part comprises in series an inverse quantization 115 followed by an inverse discrete cosine transform 116.        an intermediate branch 119 comprising a motion compensation 105 using motion vectors V(n) of the input signal, its associated memory 106 storing a previous signal, and a second subtracter 123. This branch, also called prediction loop, avoids the quality drift in the output signal by applying a motion compensation to said coding error generated during the re-encoding step.The logo addition branch 102 is implemented thanks to a residue addition to the decoded signal Error_I(n), by means of the adding sub-step 121. This residue is formed by subtracting an additional data signal Logo(n) referenced 127 with a motion-compensated logo prediction referenced 129, obtained by means of the motion compensation sub-step 103, which is based on reference pictures containing logo previously stored in memory 104 and which uses the same vectors V(n) as the main input signal.        
In the prior art diagram depicted in FIG. 1, two motion compensations are performed: a first one 105, well known and provided for correcting the quality drift on P and B pictures introduced by the quantization sub-step 111, and a second one 103 on an additional data signal 127. This motion compensation 103 generates said motion compensated signal PRED(Logo(n−1), V(n)) referenced 129 which is subtracted from said signal 127. Said motion-compensated signal is indeed essential since it cancels undesired parts of the signal relative to signal 127, previously motion-compensated by 105, in the input signal of the re-encoding step. Moreover, as a motion compensation always requires a storage of a previous signal, two memories 104 and 106 are also needed. Then, with these two motion compensation operations and two memory blocks, this solution remains not only complex as regards the CPU burden, but also expensive as regards storage memory.